The present invention relates to a color data conversion apparatus for converting first color data included in a first color gamut of color image equipment such as a CRT, according to prespecified target color data within a second color gamut of another color image equipment such as a printer, to second color data within the second color gamut as well as to a color data conversion method and a recording medium for the same. More particularly, this invention relates to a color data conversion apparatus enabling efficient performance of color data conversion without increase in its lightness when an image fetched through a scanner is displayed on a CRT or a result of an image printed by a printer is reproduced on a CRT as well as to a color data conversion method and a recording medium for the same.
Conventionally, color image equipment such as a printer and a CRT has, in many cases, a different color gamut. Therefore, there may arise a case where a color capable of being displayed on a CRT can not be printed. Disclosed in Japanese Patent Laid-Open Publication No. SHO 60-105376 (Conventional technology 1) is a color image output device for converting color data included in the first color gamut dependent on first color input/output equipment toward an achromatic color having the same lightness as that of the first color data tobe converted to second color data included in the second color gamut dependent on second color input/output equipment.
FIGS. 13A and 13B explain the concepts of conventional color data conversion for converting color data included in a color gamut of a CRT to color data included in a color gamut of a printer on a L*a*b* space. An area 301 indicated by a broken line in these figures shows a color gamut of a CRT, and an area 302 indicated by a solid line shows a color gamut of a printer In this L*a*b* space, lightness becomes higher as an L* value is larger, chroma becomes higher as values of a* and b* become larger. Positive area of b* is mainly a yellow gamut, while negative area of b* is mainly a blue gamut.
As shown in FIG. 13A, color data 303, 306, 309, and 312 displayable on a CRT are outside of the printer color gamut, therefore the color data 303, 306, 309, and 312 cannot be printed by a printer unless they are converted to data for a printer.
Therefore, in accordance with the conventional technology 1, when a color of the color data 303 is to be converted to a color within the printer color gamut 302, by marking color data 304 for an achromatic color having the same lightness as that of the color data 303 and converting the color of the color data 303 toward the color data 304, color data 305 corresponding to the color data 303 is acquired.
Also, when a color of the color data 306 for blue with high chroma is to be converted to a color within the printer color gamut 302, by marking color data 307 for an achromatic color having the same lightness as that of the color data 306 and converting the color of the color data 306 toward the color data 307, color data 308 corresponding to the color data 306 is acquired. Similarly, the color data 309 is converted toward color data 310 to acquire color data 311, and the color data 312 is converted toward color data 313 to acquire color data 314.
As described above, in the conventional technology 1, when the color data 303, 306, 309, and 312 are to be printed, the color data 305, 308, 311, and 314 are used in place of the color data described above.
However, the conventional technology 1 has a problem (described as xe2x80x9cproblem 1xe2x80x9d hereinafter) that when the color data 303 for yellow with high chroma and the color data 306 for blue with high chroma are converted, the chroma tends to degrade more. For example, when an image of a banana or a lemon photographed by a digital camera or an image of bright-colored printed matter inputted with a color scanner is printed, chroma of yellow is largely lost, thus the image becomes unnatural.
There is another problem (described as xe2x80x9cproblem 2xe2x80x9d hereinafter) that even if the color data 315 and 317 shown in FIG. 13A are converted by using the conventional technology 1, the converted color data may be color data 316 and 318 which are outside of the printer color gamut 302.
Therefore, disclosed in a paper xe2x80x9cColour Reproduction Theory based on the Principles of Colour Sciencexe2x80x9d written by Laihanen, P. (Conventional technology 2) is a technology in which all of the first color data included in the first color gamut dependent on the first color input/output equipment is converted toward an achromatic color having an intermediate lightness value between the highest lightness value and the lowest lightness value of the second color gamut to be converted to the second color data included in the second color gamut dependent on the second color input/output equipment.
FIG. 13B explains the concepts of color data conversion based on the conventional technology 2. As shown in this figure, in the conventional technology 2, color data 326 and 328 which are outside of the printer color gamut 302 are converted toward color data 322 for an achromatic color having an intermediate lightness value between the highest lightness value and the lowest lightness value of the printer color gamut 302, and color data 327 and 329 are respectively acquired.
Namely, in the conventional technology 2, the color data 322 is insured to always exist within the printer color gamut 302. Therefore any color can be printed by a printer, which is different from the conventional technology 1. Thus the problem 2 is solved. In addition, with the conventional technology 2, high-chroma yellow color data 321 is converted to color data 323, and high-chroma blue color data 324 is converted to color data 325, therefore decrease in chroma is reduced as compared to that of the conventional technology 1. Thus the problem 1 is solved.
When the conventional technology 2 is used, however, there comes up an extremely significant problem (described as xe2x80x9cproblem 3xe2x80x9d hereinafter) that each relation of both lightness values and chroma values between two color data is concurrently reversed respectively, which is called, reversion of gray scale. Especially, the reversion of gray scale described above becomes sever when a relation between a lightness value A of color data before conversion, a lightness value B of the color data 332 which is at the same hue angle as that of the color data and also has the highest chroma included in the printer color gamut 302, and a lightness value C of the color data 322 for an achromatic color having the intermediate lightness value is A greater than B greater than C or A less than B less than C.
For example, referring to the color data 330 and the color data 321 shown in FIG. 13B, original lightness and chroma values of the color data 321 are higher than the lightness value and chroma value of the color data 330. However, if the color data 331 to which the color data 330 is converted and the color data 323 to which the color data 321 is converted are compared, then it can be understood that the lightness value and chroma value of the color data 331 are higher than the lightness value and chroma value of the color data 323. Thus, a reversion of gray scale has occurred.
In the Japanese Patent Laid-Open Publication No. HEI 10-8865 applied on Jan. 20, 1998 by the applicant of the present application (conventional technology 3) a color data conversion method has been disclosed for converting first color data included in the first color gamut dependent on the first color input/output equipment toward third color data locating on a line linking color data which is at the same hue angle as that of the first color data and also has the highest chroma of the colors included in the second color gamut dependent on the second color input/output equipment to color data for an achromatic color which is at the same hue angle as that of the color data to compute second color data included in the second color gamut.
FIGS. 14A and 14B explain the concepts of color data conversion based on the conventional technology 3 and the problems therein. As shown in FIG. 14A, in the conventional technology 3, color data 360 which is at the same hue angle as that of color data 343, 349, and 352 within the CRT color gamut 301 and also has the highest chroma of the printer color gamut 302 is linked with a color data 361 for an achromatic color having the same lightness as that of the color data 360 with a line (corresponding to a dashed line 341 in FIG. 14A). Then, the color data 343, 349, and 352 as targets for conversion are converted toward color data 344, 350, and 353 located on this line, and color data after conversion 345, 351, and 354 are acquired.
Namely, in the method based on the conventional technology 3, the color data 343 is converted to color data which has higher chroma when the chroma value of the color data 343 before conversion is higher and also has higher chroma when the lightness value of the color data 343 is closer to the lightness value of the color data 360.
Similarly, color data 362 which is at the same hue angle as that of color data 346 and 355 and also has the highest chroma of the printer color gamut 302 is linked with a color data 363 for an achromatic color having the same lightness as that of the color data 362 with a line (corresponding to a dashed line 342 in FIG. 14A). Then, the color data 346 and 355 are converted toward color data 347 and 356 located on the line, and color data after conversion 348 and 357 are acquired.
As described above, with the conventional technology 3, color data with higher chroma is converted toward color data with higher chroma on the line, therefore the problem (problem 1) in the conventional technology 1 and conventional technology 2 that chroma in color data decreases can be resolved, and also the problem (problem 2) that there exists color data incapable of being converted can be resolved. Furthermore, the problem (problem 3) that lightness values and chroma values of two color data are reversed can be resolved, so that extremely favorable color reproduction can be performed when a color displayed on a CRT is to be printed by a printer.
Even if the conventional technology 3 is used, however, when a color image fetched with a color scanner is reproduced on a CRT display or when a result of an image printed by a printer is reproduced on a CRT display, there comes up a problem that the lightness of the color data after the conversion increases.
FIG. 14B explains this new problem when the conventional technology 3 is used. For convenience in description, herein the problem is explained with reference to a L*a* cross section of the L*a*b* space. It is assumed that an area 371 indicated by a broken line in the figure shows a color gamut of a CRT and an area 372 indicated by a solid line shows a color gamut of a color printer in a silver-salt photographic system. An a*-positive area is an area of mainly magenta, and an a*-negative area is an area of mainly green.
In FIG. 14B, a dashed line 374 links a color data 384 which is at the same hue angle as that of a color data 381 and also has the highest chroma of the CRT color gamut 371 with a color data 385 for an achromatic color having the same lightness as that of the color data 384. Further, a dashed line 373 links a color data 386 which is at the same hue angle as that of color data 375 and 378 and also has the highest chroma of the CRT color gamut 371 to color data 387 for an achromatic color having the same lightness as that of the color data 386.
In the conventional technology 3, the color data 375 is converted toward color data 376 on the dashed line 373 calculated so that when a chroma value of the color data 375 is higher, color data will have higher chroma and the color data also will have higher chroma when a lightness value of the color data 375 is closer to the color data 386, and then color data 377 is acquired. The color data 378 is converted to color data 379, and the color data 381 is converted to color data 383.
As described above, with the conventional technology 3, when the color data 375 and 378 are converted to color data 377 and 379, the lightness values of the data largely increase. Therefore, when the color data after the conversion is displayed on a CRT, the colors are changed to bright colors which are completely different from the original color data.
The reason why the lightness value increases is, the CRT color gamut 371 forming color by light emission is substantially different from a shape of the color gamut 372 of a color printer forming color by reflection of illuminating light. More specifically, in the CRT, the lightness value of the color data 386 with the highest chroma in an area of green color is a quite high of around 80 to 90, and as a result, the line 373 indicated by a dashed line locates in a level of high lightness. On the contrary, in the color printer, the lightness value of the color data 388 with the highest chroma in the area of green color is a comparatively low of around 40 to 50, and as a result, each direction of conversion of the color data 375 and 378 is substantially parallel with an achromatic color axis, so that the lightness value increases largely.
Thus, in the conventional technology 3, there comes up a problem that when an image fetched through a scanner is displayed on a CRT or a result of an image printed by a printer is reproduced on a CRT, the lightness of color data with low lightness located in the green area largely increases. The result is that the color of the image printed by the printer is different from the color displayed on the CRT.
The present invention has been made for solving the problems described above, and it is an object of the present invention to provide a color data conversion apparatus enabling efficient performance of color data conversion without increase in its lightness when an image fetched through a scanner is displayed on a CRT or a result of an image printed by a printer is reproduced on a CRT as well as to a color data conversion method and a recording medium for the same.
In order to achieve the object described above, the color data conversion apparatus according to the invention acquires third color data which is at the same hue angle as that of first color data within a first color gamut and also has the highest chroma within the second color gamut as well as fourth color data for an achromatic color having the same lightness as that of the third color data by an acquiring unit; and sets target color data on a first line linking the acquired fourth color data to color data corresponding to a black color in the second color gamut, on a second line linking the fourth color data to color data corresponding to a white color in the second color gamut, or on a third line linking the fourth color data to the third color data; and a converting unit converts the first color data to second color data according to the set target color data, so that the first, second, and third lines are insured to be included in the second color gamut, thus the first color data being converted to the second color data without fail. Further, the first color data can be converted to the second color data without fail. In addition, by converting color data toward target color data on the three lines of the first, second, and third lines, it is possible to realize an operation of efficiently converting color data to appropriate one without increase in its lightness when an image fetched through a scanner is displayed on a CRT or a result of an image printed by a printer is reproduced on a CRT.
The color data conversion method according to the invention comprises the steps of acquiring third color data having the highest chroma within the second color gamut as well as fourth color data for an achromatic color having the same lightness as that of the third color data each at the same hue angle as that of first color data within a first color gamut; setting target color data on a first line linking the acquired fourth color data to color data corresponding to a black color in the second color gamut, on a second line linking the fourth color data to color data corresponding to a white color in the second color gamut, or on a third line linking the fourth color data to the third color data; and converting the first color data to the second color data according to the set target color data. Therefore, the first, second, and third lines are insured to be included in the second color gamut, thus the first color data being converted to the second color data without fail. Further, the first color data can be converted to the second color data without fail. In addition, by converting color data toward target color data on the three lines of the first, second, and third lines, it is possible to realize an operation of efficiently converting color data to appropriate one without increase in its lightness when an image fetched through a scanner is displayed on a CRT or a result of an image printed by a printer is reproduced on a CRT.
The recording medium according to the invention makes a computer execute the sequence of the steps of acquiring third color data having the highest chroma within the second color gamut as well as fourth color data for an achromatic color having the same lightness as that of the third color data each at the same hue angle as that of the first color data; and setting target color data on a first line linking the acquired fourth color data to color data corresponding to a black color in the second color gamut, on a second line linking the fourth color data to color data corresponding to a white color in the second color gamut, or on a third line linking the fourth color data to the third color data, and converting the first color data to the second color data according to the set target color data. Therefore, the color data conversion method according to the invention can be realized on a computer.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.